Lactation

 Lactation is the production of milk in the mammary glands. It begins toward the end of pregnancy and is nearly in full swing at the time of parturition. Several hormones are involved in lactation - principally prolactin.

Colostrum is the initial secretion of the mammary glands after parturition. If differs from normal milk in its composition and appearance. Its most important role is to transfer antibodies (passive immunity) to the newborn. Since the gut of the newborn can only absorb the large antibody molecules for a short time, it is important that the newborn drink colostrum during the first 12-24 hours after birth.

Lactation is maintained by continued suckling or milking. Sensory stimulation of the teat or nipple causes continued production of the hormones necessary to continue lactation. If stimulation ceases, the production of milk ceases - the gland "dries up."

Milk letdown is the immediate effect of suckling or milking. When milk is produced, it stays up in the alveoli and small ducts where it is not available for milking or suckling. Stimulation of the teat or nipple causes the release of oxytocin from the posterior pituitary gland. Oxytocin causes myoepithelial cells around the alveoli and small ducts in the mammary glands to contract, forcing milk down into the large ducts and sinuses. From there it is available for suckling or milking.